.\" $OpenBSD: arc4random.3,v 1.37 2019/09/29 16:30:35 jmc Exp $
.\"
.\" Copyright 1997 Niels Provos <provos@physnet.uni-hamburg.de>
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.\" Manual page, using -mandoc macros
.\"
.Dd $Mdocdate: September 29 2019 $
.Dt ARC4RANDOM 3
.Os
.Sh NAME
.Nm arc4random ,
.Nm arc4random_buf ,
.Nm arc4random_uniform
.Nd random number generator
.Sh SYNOPSIS
.In stdlib.h
.Ft uint32_t
.Fn arc4random "void"
.Ft void
.Fn arc4random_buf "void *buf" "size_t nbytes"
.Ft uint32_t
.Fn arc4random_uniform "uint32_t upper_bound"
.Sh DESCRIPTION
This family of functions provides higher quality data than those
described in
.Xr rand 3 ,
.Xr random 3 ,
and
.Xr rand48 3 .
.Pp
Use of these functions is encouraged for almost all random number
consumption because the other interfaces are deficient in either
quality, portability, standardization, or availability.
These functions can be called in almost all coding environments,
including
.Xr pthreads 3
and
.Xr chroot 2 .
.Pp
High quality 32-bit pseudo-random numbers are generated very quickly.
On each call, a cryptographic pseudo-random number generator is used
to generate a new result.
One data pool is used for all consumers in a process, so that consumption
under program flow can act as additional stirring.
The subsystem is re-seeded from the kernel
.Xr random 4
subsystem using
.Xr getentropy 2
on a regular basis, and also upon
.Xr fork 2 .
.Pp
The
.Fn arc4random
function returns a single 32-bit value.
.Pp
.Fn arc4random_buf
fills the region
.Fa buf
of length
.Fa nbytes
with random data.
.Pp
.Fn arc4random_uniform
will return a single 32-bit value, uniformly distributed but less than
.Fa upper_bound .
This is recommended over constructions like
.Dq Li arc4random() % upper_bound
as it avoids "modulo bias" when the upper bound is not a power of two.
In the worst case, this function may consume multiple iterations
to ensure uniformity; see the source code to understand the problem
and solution.
.Sh RETURN VALUES
These functions are always successful, and no return value is
reserved to indicate an error.
.Sh SEE ALSO
.Xr rand 3 ,
.Xr rand48 3 ,
.Xr random 3
.Sh HISTORY
These functions first appeared in
.Ox 2.1 .
.Pp
The original version of this random number generator used the
RC4 (also known as ARC4) algorithm.
In
.Ox 5.5
it was replaced with the ChaCha20 cipher, and it may be replaced
again in the future as cryptographic techniques advance.
A good mnemonic is
.Dq A Replacement Call for Random .
